The largest single contributor to power losses in a magnetic bubble memory subsystem is often the circuitry required to produce the rotating magnetic drive field. These losses can be as high as 50 to 60 percent of the entire memory board power. Efforts to reduce coil drive losses, while at the same time providing transient free start/stop operation and minimizing bubble output signal noise, have led previous investigators to consider a variety of coil drive techniques. The resulting circuits can be broadly classified as either linear drive or switched drive.
The essential difference between a linear and switched coil drive circuit is that in the former circuit energy is being delivered to the coils by devices operating in their active region whereas in the latter circuit energy is delivered at only discrete time intervals by devices operating in an ON/OFF mode. In linear drive circuits the coil currents are sinusoidal and may be obtained directly from an appropriate amplifier or generated in a tuned circuit operated at resonance. In a switched drive circuit the resulting coil current may be sinusoidal, but more often it is a triangular type waveshape produced by switching alternate polarity direct voltages across the coil.
A linear drive circuit can be designed to operate at relatively high efficiency when driven at resonance. However, a problem with this type of circuit arrangement is that the operating power efficiency will drop drastically if the coil-capacitor combination is allowed to operate off resonance by as little as 4 or 5 percent. Operating a linear drive circuit as much as 5 percent off resonance in a manufacturable memory system is not uncommon considering the manufacturing tolerance on the bubble device coil inductance, the resonating capacitor, and other circuit parameters which effect timing.
In either the linear or the switched coil drive it is advantageous for the coil drive circuitry to be as efficient as possible so as to reduce memory system power requirements and minimize power dissipation in the bubble memory subsystem. Not only will this result in reduced subsystem thermal problems, but will also allow greater memory component packing density on subsystem circuit boards. In addition, system power losses can have a direct effect on memory system data rate by influencing the choice of rotating field frequency.
U.S. Pat. No. 3,952,292 describes a magnetic bubble memory drive circuit wherein a rotating magnetic field is induced by two out-of-phase sine wave currents which are supplied to two substantially perpendicularly positioned solenoid coils encompassing the layer in which magnetic bubbles move. Both linear and switched coil drive circuits are suggested for producing the sine wave coil currents. The patent also describes the conventional use of a capacitor connected in parallel with each of the coils in the coil drive circuit so that the resonance of the coil and capacitor may produce a sine wave coil current in response to a square wave drive voltage.
The patent stresses the fact that the speed of rotation of the magnetic field in response to forced square wave coil currents is not uniform in the coil drive circuit and that this nonuniformity causes problems in the normal operation of a magnetic bubble memory. The nonuniformity of the speed of rotation leads to a reduction in operating margins of the memory. The speed nonuniformity of the rotating field is said to be eliminated by the use of triangular or trapezoidal coil currents, rather than the square wave drive currents, along with the elimination of the use of capacitors in parallel with the coils.
The patent also indicates that the power losses in either a linear or a switched bubble memory coil drive circuit can be an important problem. The reduction of power losses in a drive circuit of a magnetic bubble memory is highly desirable. The prior art, recognizes that if the coil drive circuit is capable of start, stop and reverse field operation then the memory system access time can be reduced. This, in turn, may reduce the amount of time the coil drive circuit is in operation each time the memory is accessed. The corresponding reduction in memory duty cycle leads to a reduction in total system power losses.
It is important to recognize, however, that power losses in a switched coil drive circuitry, during the time it is driving the bubble device coils, are made up of two basic components. These are, first, conduction losses in the switching transistor and other devices in the switching circuit in the time interval during which they conduct current, and, second, switching losses principally in the switching transistors devices which occur during the interval of time they are switching from on to off or from off to on. Despite the fact that these switching times tend to be short compared to conduction intervals, the switching losses in a switched coil drive circuit can be a substantial proportion of the total drive losses. Power losses in a linear coil drive circuit are composed entirely of conduction losses. The problem to which the present invention is directed is the reduction of both switching and conduction losses in a switched coil drive circuit.